This invention relates to a fuel injection valve for use in internal combustion engines, and more particularly to a fuel injection valve of this kind which has a reduced nozzle needle lift during low speed/low load operation of the engine such as idling.
A diesel engine in general is provided with fuel injection valves with their nozzle tips projected into respective cylinders of the engine to inject fuel delivered from a fuel injection pump into the respective cylinders in an intermittent manner. Conventionally, automatic injection valves are employed in most diesel engines, which are constructed such that when the pressure of pressurized fuel delivered from a fuel injection pump surpasses the setting load of a nozzle spring urging a nozzle needle in its closing diretion, the nozzle needle is lifted by the pressurized fuel so that the fuel is injected into an engine cylinder.
However, according to such conventional construction, it is difficult to control both the fuel injection rate and the fuel injection period to respective different values according to operating conditions of the engine, particularly, between during low speed/low load operation of the engine such as idling and during normal operation of same. Further, in the conventional automatic injection valves, the nozzle needle is lifted through a constant stroke over all operating conditions of the engine. As a result, the conventional construction has the disadvantage that during low speed/low load operation of the engine such as idling fuel injection takes place over a very short period of time and at a high injection rate, causes large combustion noise in the engine.
To overcome this disadvantage, a fuel injection valve has been proposed, e.g. by Japanese Provisional Utility Model Publication No. 59-62273, which comprises a main body having injection holes formed in its tip and a fuel intake passageway formed therein and communicating with the injection holes, a nozzle needle slidably fitted within the main body for alternately closing and opening the injection holes, a lift-limiting plunger slidably fitted in a plunger chamber defined within the main body and having an end face remote from the nozzle needle disposed as a pressure-receiving surface to receive fuel pressure for limiting the lifting amount of the nozzle needle, and a selector valve disposed to selectively establish or interrupt communication between a back pressure chamber defined by the end face of the plunger remote from the nozzle needle as part of the plunger chamber and the above-mentioned fuel intake passageway. During low speed/low load operation of the engine, the selector valve is controlled to establish communication between the back pressure chamber and the fuel intake passageway, whereby the the nozzle needle is lifted through a limited or reduced stroke to obtain a reduced fuel injection rate as well as a prolonged fuel injection period.
According to this proposed fuel injection valve, by virtue of the limited lifting stroke of the nozzle needle the opening area of the injection holes is kept at a moderate small value to obtain a reduced injection quantity, that is, a lower fuel injection rate and a longer injection period, even when the pressurized fuel from the fuel injection pump has high pressure, thereby mitigating engine combustion noise during low speed/low load operation of the engine such as idling.
However, according to the proposed fuel injection valve, if during needle lift-limited operation a pumping plunger of the fuel injection pump is operated through the same effective delivery stroke as that executed by a plunger of a conventional ordinary type fuel injection valve incapable of limiting the lifting amount of the nozzle needle, the actual quantity of fuel injected into the engine cylinder decreases by an amount corresponding to the amount of throttling the injection holes by the limited stroke of the nozzle needle, resulting in reduced engine output.
One way to prevent such engine output reduction would be to set a control sleeve of the fuel injection pump to a position where the effective delivery stroke of the nozzle needle is increased during needle lift-limited operation so as to compensate for a reduction in the fuel injection quantity caused by limiting the lifting amount. However, this requires returning the control sleeve to a normal position when the engine enters a normal operating region, which in turn requires a complicate control mechanism as well as a complicate manner of operating the mechanism. Besides, it is also difficult to smoothly change the effective delivery stroke of the pumping plunger at changeover from needle lift-limited operation to needle lift-unlimited operation or vice versa.